1. Field of the Invention
The present invention relates to a hydraulic control valve used for construction machinery such as a hydraulic shovel, and more particularly, to structure of a check valve incorporated in the hydraulic control valve.
2. Description of the Related Art
Construction machinery, for example, a hydraulic shovel normally comprises hydraulic cylinders for driving a boom, an arm, a shovel, and the like, and hydraulic motors for driving a swivel base, a travelling caterpillar, or a vehicle. To each of the hydraulic actuators, a required pressure oil is supplied from hydraulic pumps via each of hydraulic control valves of a control valve unit mounted on the swivel base.
Incidentally, as illustrated in FIG. 1 of JP 2002-155903 A (hereinafter, referred to as Patent Document 1), the control valve unit is provided with switching valves k1, k2, and k3 connected in tandem via a center bypass path 34. The switching valves k2 and k3 are provided with check valves via a parallel line 32. Further, on a downstream side of each of the check valves, each of the switching valves k2 and k3 is provided with another check valve provided between each of the check valves and the center bypass path 34.
FIG. 2 of the present application refers to a control valve 6 which is a hydraulic circuit part of FIG. 1 of the Patent Document 1 described above, and simply illustrates only a part relevant to the invention of the present application. Note that, in the following description, a component corresponding to the switching valve in the Patent Document 1 is referred to as a hydraulic control valve, and a component corresponding to the control valve therein is referred to as a control valve unit.
In FIG. 2, three hydraulic control valves VL1 to VL3 are connected in tandem to each other. Apart of pressure oil delivered from a variable delivery pump VPM is provided to the tandem path 2, another part thereof passes a bypass path 1, a throttle 3 and a check valve 5 and, merges with the part of pressure oil having passed a check valve 4 in parallel provided in a path branched from the tandem path 2, with the result of being fed to a port PT2 of the hydraulic control valve VL2. The hydraulic control valve VL3 has the similar circuit configuration.
The configuration of FIG. 2 enables simultaneously driving of a low-load hydraulic actuator and a high-load hydraulic actuator. The throttle 3 is adjusted in accordance with a magnitude of a load. Further, the check valve 4 prevents backflow into the tandem path 2, and the check valve 5 prevents backflow into a parallel path 1.
FIG. 3 illustrates a main part of the structure of the hydraulic control valve VL2. In FIG. 3, in the hydraulic control valve VL2, a spool Spr is arranged in a hole passing though the inside of its casing CS. Reference symbols a and b shows pressure-oil ports for controlling the position of the spool Spr. Paths i and ii illustrated in the center of the figure constitute branch paths for supplying pressure oil supplied from the tandem path 2 or the bypass path 1 originally from a pump in accordance with the position of the spool Spr into ports APT or BPT. The branch operation of the pressure oil is performed by a check valve unit CH.
In the check valve unit CH, a main unit 6 thereof has an upper portion provided with the throttle 3, a lower portion provided with an opening portion c, and an outer peripheral portion provided with a threaded portion d for attachment to the casing CS.
The opening portion c in the lower portion of the check valve unit CH houses the first check valve 4 facing the tandem path 2 and having a downward first valve body 4a seating on a seating surface of the casing CS in the illustration, and the second check valve 5 having an upward second valve body 5a slidably housed in the first check valve 4. That is, in order to achieve compactification of structure as a hydraulic control valve, it has a constitution in which two check valves are coupled to each other.
Note that, each of the valve bodies 4a and 5a are urged downward and upward by springs (not shown).
As illustrated in the cross-section A-A, the first check valve 4 has an outer periphery substantially quadrangular in cross-section. Reference symbol 4b between the first check valve 4 and the cylindrical lower portion of the check valve unit CH constitutes a path for leading the pressure oil from the bypass path 1 into the path i or ii.
However, the conventional structure illustrated in FIG. 3 has the following problems.
That is, imbalance of a fluid force around the first check valve 4 causes the first check valve 4 to rotate at high speed due to a phase shift accompanying errors in machine processing on the throttle 3 or due to a flow-path shape on a downstream side of the first check valve 4, further the first valve body 4a is pressed against the seating surface of the casing CS, thus there has occurred a phenomenon that pressure oil passing through the path 4b abrades the casing itself. Since the casing CS is made of a cast metal and the valve body 4a is made of a steel material, the abrasion progresses to a lower side in FIG. 3 especially in a state in which an oil film on the seating surface has run out, and it may lead to a risk of destruction of a regular function as a hydraulic control valve. Further, the high-speed rotation of the first valve body 4a wastes energy of pressure oil, with the result that it may cause hydraulic oil to heat.